H2-dominated Atmosphere as an Indicator of Second-generation Rocky White Dwarf Exoplanets

Zifan Lin,Lisa Kaltenegger,Sukrit Ranjan,Thea Kozakis,Sara Seager

doi:10.3847/2041-8213/ac4788

Abstract

Abstract Following the discovery of the first exoplanet candidate transiting a white dwarf (WD), a “white dwarf opportunity” for characterizing the atmospheres of terrestrial exoplanets around WDs is emerging. Large planet-to-star size ratios and hence large transit depths make transiting WD exoplanets favorable targets for transmission spectroscopy; conclusive detection of spectral features on an Earth-like planet transiting a close-by WD can be achieved within a medium James Webb Space Telescope program. Despite the apparently promising opportunity, however, the post-main sequence evolutionary history of a first-generation WD exoplanet has never been incorporated in atmospheric modeling. Furthermore, second-generation planets formed in WD debris disks have never been studied from a photochemical perspective. We demonstrate that transmission spectroscopy can identify a second-generation rocky WD exoplanet with a thick (∼1 bar) H2-dominated atmosphere. In addition, we can infer outgassing activities of a WD exoplanet based on its transmission spectra and test photochemical runaway by studying CH4 buildup.

Highlights

An exciting opportunity for characterizing the atmospheres of terrestrial exoplanets transiting white dwarf (WD) is emerging
Large planet-to-star size ratios and large transit depths make transiting WD exoplanets favorable targets for transmission spectroscopy – conclusive detection of spectral features on an Earthlike planet transiting a close-by WD can be achieved within a medium James Webb Space Telescope (JWST) program
The key difference between these two high mean molecular weight (MMW) atmospheric compositions and the H2-dominated scenario is that a WD exoplanet with a high MMW atmosphere can either be first- or second-generation

Summary

Introduction

An exciting opportunity for characterizing the atmospheres of terrestrial exoplanets transiting WDs is emerging. Kaltenegger, MacDonald et al (2020) explored the possibility of observing transiting Earth-like WD planets with JWST and described a “white dwarf opportunity” of detecting biosignature gases on such planets. For a hypothetical Earthsized planet with Earth-like atmosphere transiting WD 1856+534, JWST can detect H2O and CO2 with just a few transits and detect biosignature gases such as the O3 + CH4 pair in 25 transits (Kaltenegger, MacDonald et al 2020). JWST would struggle to detect the O3 + CH4 biosignature pair on a terrestrial planet orbiting a M dwarf such as TRAPPIST-1e even with 100 transits (e.g., Lin et al 2021)

Methods

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Discussion

Conclusion